1. Field of the Invention
The present invention relates to thin film transistors in a flat panel display. More particularly, the present invention relates to a novel structure for the electrodes and conductive lines found in thin film transistor structures that have small resistance loss and prevent image quality degradation caused by resistance loss in a large flat panel display.
2. Description of the Related Art
A thin film transistor (hereinafter, referred to as TFT) is a device of where a source electrode and a drain electrode can be electrically connected through a channel formed in a semiconductor layer that physically connects the source and drain electrodes according to a voltage applied to a gate electrode. The TFT is mainly used in a TFT panel of an active matrix flat panel display such as an electroluminescent display and a liquid crystal display. The TFT serves to independently drive sub-pixels that make up the display.
A source electrode and a gate electrode of a TFT formed in a flat panel display are connected to driving circuits arranged on sides of the flat panel display through conductive lines. Generally, a source electrode, a drain electrode, and conductive lines electrically connected to the source and drain electrodes are often formed together and have the same structure using the same material for the sake of simplifying a manufacture process.
A source electrode, a drain electrode, and conductive lines electrically connected thereto may be made of a chromium (Cr) based metal or a molybdenum (Mo) based metal such as Mo and MoW. However, since these metals have a relatively high resistance, in a case where a TFT panel has a large size or its sub-pixels have small sizes, a voltage drop between driving circuits and the sub-pixels may increase. This causes a response speed of the sub-pixels to decrease or to result in a non-uniform distribution of an image. These problems of speed and non-uniformity of image are further aggravated by the fact that newer displays are large, and the conductive lines that electrically connect to the pixels in aq large display are very long. These long conductive lines found in large displays magnify the resistive losses in a conductive line. Therefore, in recent years, with the advent of large TFT displays, it is even more important to use materials with low resistive losses to electrically connect to tach TFT in a display.
In addition to the fact that newer displays are large, the speed and non-uniformity problem are further aggravated by the fact that these conductive lines generally undergo a heat treatment process subsequent to formation of these conductive lines and electrodes. For example, the activation process after gate metal sputtering is necessary in TFT fabrication, and the temperature of annealing is generally needed to be higher than 400° C. In this case, the high temperature annealing may cause connection lines and electrodes to form at a high resistance, especially when incorporated in a large display panel.
In order to solve the above problems, aluminum (Al) has been used in conductive lines and electrode structures for TFT's. Aluminum may have a low resistance as a material for a gate electrode and a conductive line connecting the gate electrode to a driving circuit. Aluminum may also drop the resistance in source electrodes, drain electrodes, and conductive lines electrically connected to the source and drain electrodes. Hereinafter, the source electrode, the drain electrode, the gate electrode and the conductive lines electrically connected to the source, drain and gate electrodes will be referred to as “TFT conductive elements.”
U.S. patent application Laid-Open Publication No. 2002/0085157 to Tanaka et al (hereinafter Tanaka '157) discloses TFT conductive elements made of Al. Each of the TFT conductive elements has a stacked structure of titanium nitride (TiN) layer/Al layer, TiN layer/Ti layer/Al layer, or TiN layer/Al layer/Ti layer, as illustrated in FIG. 7 of Tanaka '157. Advantages of such a structure include reduction of an electrical connection resistance (or contact resistance) between the TFT conductive elements and terminals connected to the TFT conductive elements as well as suppression of the generation of Al hillocks (or small hills or mounds) generated by a heat treatment process subsequent to the formation of the TFT conductive elements. However, Tanaka '157 fails to disclose solutions to reduce the resistance of the TFT conductive elements. Tanaka '157 fails to address prevention of the formation of highly resistive TiAl3 when heat treated. TiAl3 in the conductive layers causes the resistance of the conductive lines to increase, especially for large displays.
What is therefore needed is a structure for conductive lines as well as structures for electrodes in TFT's for a large display that have a low resistance, even after a heat treatment, the conductive lines and electrodes do not have TiAl3 present and have no hillocks even after heat treatment.